In driving a rotary electric machine by an inverter, a control mode is switched over among a sinusoidal wave current control mode, an overmodulation current control mode and a rectangular wave voltage phase control mode. The rectangular control mode, which uses one-pulse switching, is required to attain both high output power and size reduction of the rotary electric machine. The rotary electric machine is controlled optimally by switching over the control mode between the sinusoidal wave current control mode, which provides a superior characteristic in a low speed region, and the overmodulation current control mode, which is used in an intermediate speed region.
In the sinusoidal wave current control mode and the overmodulation current control mode, current feedback control is performed to output a pulse-width modulation (PWM) pattern to the rotary electric machine by comparing a command voltage and a carrier wave voltage. In the rectangular wave voltage phase control mode, torque feedback control is performed to output a one-pulse switching waveform to the rotary electric machine in accordance with an electric angle, while fixing an amplitude of voltage to a maximum value and controlling phase.
The switching from the sinusoidal wave current control mode to the overmodulation control mode, from the overmodulation current control mode to the rectangular wave voltage phase control mode is performed in accordance with a rate of modulation or an amplitude of a command voltage corresponding to the modulation rate. The switching from the rectangular wave voltage phase control mode to the overmodulation current control mode is performed by checking a switching time point in accordance with a phase of an actual current (instantaneous current) relative to a command current, because the amplitude of the command voltage is fixed in the rectangular wave voltage phase control mode.
According to an AC motor drive control system disclosed in the following patent document 1, a control mode is switched over from a PWM current control mode to an overmodulation control mode when an amplitude of a voltage exceeds 1.00 time of a peak value of a reference triangular wave voltage. The control mode is switched over to a rectangular wave voltage phase control mode when the amplitude of the voltage exceeds 1.27 times of the peak value of the reference triangular wave voltage. The control mode is switched over from the rectangular wave voltage phase control mode to the overmodulation current control mode when an absolute value of phase of an actual current decreases to be less than an absolute value of phase of a command current.
Low-pass filtering is performed on a measured current, because a d-axis current and a q-axis current include periodic noises and high harmonic components. Due to this filtering, the switching-over from the rectangular wave voltage phase control mode to the overmodulation current control mode is sometimes delayed. This delay causes hunting of current phase and unstable control. Therefore, the delay in switching-over from the rectangular wave voltage phase control mode to the overmodulation current control mode is reduced by comparing a required voltage amplitude VR taking into consideration of induced voltages and the like with the peak value of the reference triangular wave voltage and providing an offset value for preventing chattering.
According to a motor drive system disclosed in the following patent document 2, an excessive voltage is generated inside the motor drive system due to excessive regenerative electric power, when an AC motor generates excessive electric power, which exceeds a level acceptable at a DC power source side, that is, an input side. Therefore, electric power consumption of the AC motor is increased to suppress the regenerative electric power supplied to the input side. In PWM control, an optimum efficiency characteristic line, which connects maximum efficiency points in relation to current phase and output torque, is determined with respect to a current amplitude. A loss increase characteristic line is determined as a set of current operation points, which are current phase-shifted from the optimum efficiency characteristic line, so that the control is performed on the loss increase characteristic line. In a rectangular wave voltage control, a supply voltage to a motor is controlled in respect of only phase and a current phase is fixed. Therefore, a required voltage for the motor is decreased by setting the current operation point to an advanced side in a relation of a supply voltage to a motor (motor interphase voltage) and a current phase in respect of torque, so that motor current is controlled in accordance with the PWM control. Thus, motor drive efficiency is decreased while ensuring torque controllability.
According to an electric motor control system, which can stably controls a motor even in transient time of rapid changes in a command torque, rotation speed and the like, disclosed in the following patent document 3, PWM control and rectangular voltage control are switched over. The PWM wave voltage control is performed when an absolute value of a command voltage of each phase of an AC motor is less than A/2 with “A” being a value equivalent to a battery voltage. The rectangular wave voltage control is performed when the absolute value of the command voltage is between A/2 and A/2×4/π. The value of the command torque is decreased when the absolute value of the command voltage is more than a maximum voltage value.    Patent document 1: JP 2008-11682A (WO 2008/001524A1)    Patent document 2: JP 2007-151336A (EP 1950878A1)    Patent document 3: JP 2000-358393A
As described above, the control mode is switched over from the rectangular wave voltage phase control mode to the overmodulation current control mode by determining the switching time point in accordance with the current phase of the actual current relative to the command current. If the switching time point is determined in the advance side in a d-q plane relative to the command current as a reference, the control chatters and current fluctuates at the time of switching over the control mode. For this reason, the switching time point is determined at a retard side in the d-q plane relative to the command current as the reference.
Because the rectangular wave includes high harmonic components, chattering is caused if the switching time point is determined in accordance with an actual current, and the current is disturbed by a step change of a voltage caused by dead-time and the like. Therefore, the switching time point is determined by using, not the actual current but a smoothed current, which is produced by filtering the actual current in an arbitrary predetermined time constant.
As a result, in a transitional state in which a command torque and a rotation speed fall rapidly, a current phase difference between the actual current and the smoothed current possibly becomes large. That is, although it is already a time point of switching over the control mode in the case of control in accordance with the instant current, it is determined to be still in a region of the rectangular wave voltage phase control mode in the case of control in accordance with the smoothed current. Thus, if the switching-over of the mode is delayed, the d-axis actual current jumps over the operation region and causes no torque generation.
For example, in the case of a rotary electric machine having salient poles and having no salient poles, the motor is operated in regions where the d-axis currents are positive and negative, respectively. No torque will be produced outside these operation regions. Therefore, when the d-axis current changes from a large negative value toward a small value in the rectangular wave voltage phase control mode in the rotary electric machine having no salient poles, the torque is not produced if the d-axis current jumps and reaches the positive region passing over the mode switching determination point.